In the manufacture of aircraft, as well as other large scale assemblies which require a large number of fasteners, bolts are frequently used to fasten two or more parts together. The assembly of an aircraft wing, involving stringers, spars and panels, is an example. In general, bolts are an alternative to rivets for fastening purposes, but are preferable to rivets and other fasteners in many situations. For instance, bolts are typically stronger than rivets, and thereby provide better overall damage protection in the event of a partially broken part. Further, bolts are typically more reliable when a relatively thick stack of parts is to be fastened.
When bolts are used to fasten two parts, such as the use of titanium bolts in an aluminum aircraft structure, they are driven through the parts to be fastened in a manner to form what is known as an interference fit, where the opening through the parts is slightly smaller (typically several thousandths of an inch) than the shank of the bolt which is to be inserted therein. To achieve an interference fit, the bolts must be driven into the opening with considerable force. Compressive stress between the bolt and the parts is produced, which typically results in increased fatigue life for the joint.
Generally, however, the current techniques for producing an interference fit in the aircraft industry have several significant disadvantages. In a first technique, a bolt is driven into the opening through the parts with a pneumatic rivet gun. Typically, a rivet gun includes a driver mass, which is moved at high velocity along a barrel by means of air pressure. The mass strikes an anvil which in turn hits the head of the bolt, which has been previously positioned in the opening, with several sharp blows, until the bolt is fully inserted. Pneumatic guns, however, are very loud and are labor-intensive in use.
Second, a hydraulic driver has been used to achieve an interference fit with bolts. Typically, the hydraulic driver is used with large, C-shaped riveting machines, which reach around the aircraft or other parts to be fastened. Such equipment, however, is extremely large, cumbersome to operate, and expensive. In addition, with a hydraulic driver, the shank of the bolt is put into compression, which results in the diameter of the bolt shank swelling upon insertion, which in turn increases the required insertion force. This in turn may have a negative effect on the stress condition of the bolt in the opening.
A third technique is used with a pintail-type bolt, which has a free end portion which is smaller than the remainder of the bolt. The small diameter free end of the bolt is first passed through the opening through the parts to be fastened to the point where the small end (the pintail) clears the opening. A hydraulic tool then pulls on the pintail from the rear, drawing the bolt into the opening. Special tools are necessary to clamp onto the pintail and to pull the bolt through the opening, into an interference fit. This process is expensive and is labor-intensive.
Lastly, an electromagnetic bolt insertion system is shown in U.S. Pat. No. 3,945,109 to Leftheris. However, the Leftheris system requires extremely high voltage and is not practical in operation. In the Leftheris system, the driver is initially positioned against the head of the bolt to be driven. A specially designed driver, coil and power supply are required. However, safety and reliability concerns, as well as expense, has prevented any use of such a system.